Semiconductor device having buried-type element isolation structure and method of manufacturing the same

ABSTRACT

The present invention is a semiconductor device having an element isolation structure of STI, in which after the formation of the STI trench, a silicon nitride film is left over only on the side wall portion of the trench, to form a side wall. Further, ions are implanted from the bottom surface of the trench on which the side wall is formed, and thus a high-concentration punch-through suppression region having the same conductivity as that of the substrate (or well) and a concentration higher that the impurity concentration of the other section close to the substrate (or well), is formed selectively only in the section of the substrate (or well) which is near the bottom surface of the trench. In this manner, the punch-through suppression region can be formed only in the bottom portion of the STI in a self-alignment manner by the thickness of the side wall. With this structure, even if the STI has a shallow or fine element isolation structures the punch-through between diffusion layers can be suppressed, and the occurrence of a junction leak between the high-concentration diffusion layer region and the well can be prevented. Therefore, a well structure of a low capacitance, which is suitable for a high-speed operation device, can be designed.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor device having aburied-type element isolation structure, and a method of manufacturingsuch a semiconductor device, and more specifically, to a semiconductordevice in which elements are isolated by STI (shallow trench isolation)and a method of manufacturing such a device.

As is known, semiconductor devices having a buried-type elementisolation structure entail the advantages of decreasing the size ofelement isolation regions, and achieving a well structure capable ofsuppressing the capacitance of the diffusion layer and being suitablefor high-speed operation device.

For example, in order to maintain the capacitance of the diffusion layerat low, it suffices only if the concentration of impurities in theportion of the substrate, which corresponds to the bottom surface of thediffusion layer, or the concentration in the well is set to besufficiently low. However, when the well concentration is loweredexcessively, punching through between diffusion layers becomesuncontrollable. In order to avoid this, the concentration in the portionof the substrate, which corresponds to the bottom surface of the buriedelement isolation structure, or the well concentration is selectivelyincreased, and thus the reduction of the capacitance of the diffusionlayer and the control of the punch-through between diffusion layers areachieved at the same time in conventional techniques.

However, as the semiconductor devices are downsized, the trench for theelement isolation becomes shallower (which is so-called STI). Therefore,even with the method described above, it is becoming difficult toachieve the reduction of the capacitance of the diffusion layer and thecontrol of the punch-through for the element isolation, at the sametime.

The conventional technique mentioned above will now be briefly reviewedwith reference to FIGS. 1A to 1D, which are cross sections illustratinga manufacturing step for manufacturing the conventional buried-typeelement isolation structure and drawback of such a conventionaltechnique.

First, as shown in FIG. 1A, conventionally, a silicon oxide film 2 isformed to have a thickness of about 10 nm, on a semiconductor (silicon(Si)) substrate 1 by a thermal oxidation method or the like. Then, asilicon nitride film 3 is deposited to have a thickness of about 200 nm,on the silicon oxide film 2 by a chemical vapor growth method or thelike. Further, thus resultant structure is treated in the followingmanner. That is, the silicon nitride film 3, the silicon oxide film 2and the silicon substrate 1 are subjected to anisotropic etching oneafter another by a photo-etching method. Thus, a buried type elementisolation trench 4 having a predetermined shape is made. After that,heat oxidation is carried out, and consequently, a silicon oxide film 5having a thickness of, for example, about 15 nm is formed on the innerwall of the buried element isolation trench 4.

Next, as shown in FIG. 1B, for example, boron ions are implanted to theabove-described structure at an acceleration voltage of 20 keV and aconcentration of 1×10¹³ cm⁻² in the case where the substrate (or well) 1in the region where the buried element isolation trench 4 is formed isp-type. Or, for example, phosphor ions are implanted to theabove-described structure at an acceleration voltage of 30 keV and aconcentration of 1×10¹³ cm⁻² in the case where the substrate (or well) 1in the region where the buried element isolation trench 4 is formed isn-type. Thus, in a region of the substrate (or well) 1, whichcorresponds to the bottom portion of the buried element isolation trench4, a punch-through suppression region 6 having the same conductivitytype as that of the substrate (or well) of the region and having animpurity concentration higher than that of other substrate (or well) 1located close thereto, is formed.

Further, to the structure shown in FIG. 1B, an insulating film 7 such assilicon oxide film is buried, and then the insulating film 7 isflattened by a CMP (chemical mechanical polish) method, or a resist etchback method or the like. Subsequently, the insulating film 7, thesilicon nitride film 3 and the silicon oxide film 2 are removed exceptfor the matter inside the buried element isolation trench 4, thuscompleting a buried type element isolation structure 7′ as shown in FIG.1C.

Next, as shown in FIG. 1C, for example, arsenic ions are implanted tothe above-described structure at an acceleration voltage of 40 keV and aconcentration of 3×10¹⁵ cm⁻² in the case where the substrate (or well) 1in the region where the element isolation structure 7′ is formed isp-type. Or, for example, BF₂ ions are implanted to the above-describedstructure at an acceleration voltage of 30 keV and a concentration of3×10¹⁵ cm⁻² in the case where the substrate (or well) 1 in the regionwhere the element isolation structure 7′ is formed is n-type. Thus, ahigh-concentration diffusion layer region 8 is formed in a vicinity ofthe surface portion of the substrate (or well) 1.

After that, as shown in FIG. 1D, an interlayer insulating film 10 isdeposited on the high-concentration diffusion layer region 8 and theelement isolation structure 7′, and a contact 11 designed to make anelectrical contact with the high-concentration diffusion layer region 8is formed in the interlayer insulating film 10. Further, a metal wiring12 which is connected to the contact 11 is formed on the interlayerinsulating film 10.

However, the element isolation structure 7′ thus formed entails thefollowing drawbacks.

That is, it is originally preferable that the high-concentrationdiffusion layer region 8 shown in FIG. 1C should be in contact with asubstrate (or well) 1 of a lowest possible concentration, in order tokeep the capacitance of the bottom surface at low. However, in themanufacturing step described above, the high-concentration diffusionlayer region 8 and the punch-through suppression region 6 are broughtinto contact with each other in a region 9 located close to the elementisolation structure 7′. Therefore, in the close region 9, the reductionof the capacitance cannot be realized, which is not preferable toincrease the high-speed operation of the semiconductor device.

Further, as counter-measurements, there is a method of implanting iononly to the substrate (or well) 1, which corresponds to the bottomportion of the element isolation structure 7′, in order to suppress thepunch-through. However, even in the method, impurities diffuse in thesubstrate (or well) 1 in the lateral direction. For this reason, indevices of the future, which have shallower element isolation structure7′, it becomes difficult to reduce the capacitance of the diffusionlayer.

More specifically, as the semiconductor device is downsized, thepossibility where the punch-through suppression region 6 and thehigh-concentration diffusion layer region 8 are in contact with eachother becomes higher. This is because although the high-concentrationdiffusion layer region 8 becomes thinner, the size of the punch-throughsuppression region 6 is not always reduced, in order to maintain theconcentration of the impurities in the punch-through suppression region6, which accords with the downsizing. Therefore, the high-concentrationdiffusion layer region 8 and the high-concentration punch-throughsuppression region 6 can be easily brought in contact with each other,and it becomes further difficult to form a low-capacitance diffusionlayer.

Further, as can be seen in FIG. 1D, as the downsizing proceeds, thedistance between the contact 11 used to obtain electrical contact withthe high-concentration diffusion layer region 8, and the elementisolation structure 7′ becomes shorter.

Therefore, when a mask alignment error occurs during the photo-etchingprocess, the contact 11 is overlaid upon the element isolation structure7′.

When the above-described problem occurs, the overlying section 7 a ofthe element isolation structure 7′ is etched when the contact hole forthe contact 11 is made, and thus a junction leak is created between thehigh-concentration diffusion layer region 8 and the well.

BRIEF SUMMARY OF THE INVENTION

The object of the invention is to provide a semiconductor device capableof a high performance, in which the capacitance of thehigh-concentration diffusion layer region can be suppressed at low evenfor a shallow or fine element isolation structure, and the occurrence ofa junction leak between the high-concentration diffusion layer regionand the well is prevented, and a method of manufacturing such asemiconductor device.

In order to achieve the above-described object of the present invention,there is provided a semiconductor device having a buried-type elementisolation structure, comprising: a substrate or well region, of a firstconductivity type; a buried element isolation trench formed in thesubstrate or well region of the first conductivity type; ahigh-concentration impurity region of the first conductivity type,formed in a section of the substrate or well region of the firstconductivity type, which is located near a bottom surface of theburied-type element isolation trench; an element isolation structureportion formed within the buried-type element isolation trench; adiffusion layer region of a second conductivity, formed in a surfaceportion of the substrate or well region of the first conductivity type,except for a region where the element isolation structure portion isformed; an interlayer film deposited on the substrate or well region ofthe first conductivity type; and a contact section pierced through theinterlayer film, to be connected to the diffusion layer region; whereinthe element isolation structure portion is formed by burying aninsulating film having an etching selectivity ratio to the interlayerfilm, in at least a side wall portion of the buried element isolationtrench, the high-concentration impurity region is formed selectivelylower than the bottom surface of the buried element isolation trench, ata predetermined distance from an end portion of the bottom surface ofthe buried element isolation trench, and the contact section is formedto extend over the diffusion layer region and the element isolationstructure portion.

According to the present invention, there is further provided a methodof manufacturing a semiconductor device having a buried-type elementisolation structure, including: a substrate or well region, of a firstconductivity type; a buried element isolation trench formed in thesubstrate or well region of the first conductivity type; ahigh-concentration impurity region of the first conductivity type,formed in a section of the substrate or well region of the firstconductivity type, which is located near a bottom surface of theburied-type element isolation trench; an element isolation structureportion formed within the buried-type element isolation trench; adiffusion layer region of a second conductivity, formed in a surfaceportion of the substrate or well region of the first conductivity type,except for a region where the element isolation structure portion isformed; an interlayer film deposited on the substrate or well region ofthe first conductivity type; and a contact section pierced through theinterlayer film, to be connected to the diffusion layer region; wherein,after an insulating film having an etching selectivity ratio to theinterlayer film is formed in at least a side wall portion of the buriedelement isolation trench, impurities are introduced, so as to form thehigh-concentration impurity region at an inner side from an end portionof the bottom surface of the buried element isolation trench by adistance determined by a thickness of the insulating layer, and thecontact section is formed to extend over the diffusion layer region andthe element isolation structure portion.

According to the present invention, there is still further provided amethod of manufacturing a semiconductor device, comprising: the firststep of forming an oxide film on a substrate or well region, of a firstconductivity type; the second step of forming a mask film to make aburied element isolation trench, on the oxide film; the third step ofmaking a buried element isolation trench by processing the mask film,the oxide film and the substrate or well region, with anisotropicetching; the fourth step of forming an insulating film along an innersurface of the buried element isolation trench; the fifth step offorming a high-concentration impurity region of the first conductivitytype, formed selectively in a section of the substrate or well region ofthe first conductivity type, which is located near a bottom surface ofthe buried-type element isolation trench; the sixth step of forming anelement isolation structure portion by burying a filler member in theburied-type element isolation trench; the seventh step of forming adiffusion layer region of a second conductivity, in a surface portion ofthe substrate or well region of the first conductivity type, except fora region where the element isolation structure portion is formed; theeighth step of depositing an interlayer film having an etchingselectivity ratio to the insulating film, on an entire surface; and theninth step of making a contact section pierced through the interlayerfilm, to be connected to the diffusion layer region in a self-alignmentmanner with respect to the element isolation structure portion.

With the semiconductor device and the manufacturing method of thepresent invention, it becomes possible to form an ion-implanted regionused for the punch-through suppression, selectively in the bottomsection of the STI in a self-alignment manner. More specifically, a sidewall is formed on a side surface of a trench after the formation of thetrench of the STI, and the ion implantation of impurities into thestructure is performed, thus forming a punch-through suppression region.In this manner, the punch-through control region is formed selectivelyat the bottom portion of the STI, and therefore an increase in thecapacitance of the diffusion layer, which is caused by the diffusion ofimpurities in the lateral direction, can be suppressed.

Further, when a filler member is buried into the structure, it is filledinto a front taper shape. Therefore, even if the trench itself has arectangular shape, it is possible to bury the filler member into thetrench without creating a void.

Alternatively, according to the semiconductor device and themanufacturing method, of another aspect of the present invention, afiller member is buried into a trench at a degree about equivalent tothe amount of the ion implantation to the bottom portion, and impuritiesare ion-implanted to the bottom portion of the STI. After that, thetrench is completely filled with the filler member. With this structure,it becomes possible to provide an offset in a self-alignment manner withrespect to the side wall of the STI. Further, the offset serves as abuffer while the impurities implanted to the bottom portion of the STIdiffuse, and thus it is possible to prevent the diffusion of theimpurities to the element region.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIGS. 1A to 1D are cross sectional views illustrating steps of themanufacturing process for preparing a conventional buried elementisolation structure;

FIGS. 2A to 2F are cross sectional views illustrating steps of themanufacturing process for preparing a semiconductor device, according tothe first embodiment of the present invention, taking the example wherethe product is a logic element;

FIG. 3 is a plan view briefly showing the semiconductor device shown inFIGS. 2A to 2F;

FIG. 4 is a plan view briefly showing a semiconductor device accordingto the second embodiment of the present invention, taking the examplewhere the product is an SRAM (static random access memory);

FIGS. 5A to 5E are cross sectional views illustrating steps of themanufacturing process for preparing the SRAM shown in FIG. 4;

FIG. 6 is a cross section briefly showing the SRAM shown in FIG. 4,taken along the line VI—VI; and

FIGS. 7A to 7D are cross sectional views illustrating steps of themanufacturing process for preparing a semiconductor device, according tothe third embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will now be described in detailwith reference to accompanying drawings.

(First Embodiment)

FIGS. 2A to 2F briefly illustrate the manufacturing process for making asemiconductor device according to the first embodiment of the presentinvention, taking the example where the product is a logic element.

First, as shown in FIG. 2A, a silicon oxide film 22 is formed to have athickness of about 10 nm, on a semiconductor substrate 21 by a thermaloxidation method or the like. Then, a silicon nitride film (mask film)23 is deposited to have a thickness of about 200 nm, on the siliconoxide film 22 by a chemical vapor growth method or the like.

With respect to the above-described structure, the silicon nitride film23, the silicon oxide film 22 and the semiconductor substrate 21 aresubjected to anisotropic etching one after another by a photo-etchingmethod. Thus, a buried type element isolation trench 24 having a taperedshape with a predetermined angle, which is a trench used for isolatingthe buried type elements, is made. After that, heat oxidation is carriedout, and consequently, a silicon oxide film 25 having a thickness of,for example, about 15 nm is formed on the inner wall of the buriedelement isolation trench 24.

Next, with regard to the resultant structure shown in FIG. 2A, a siliconnitride film 26 is deposited to have a thickness of about 50 nm, on thesilicon nitride film 23 and the silicon oxide film 25, by a chemicalvapor growth method, as illustrated shown in FIG. 2B. The siliconnitride film 26 becomes to have an etching selection ratio with respectto the silicon oxide film 25 or an interlayer insulating film, when acontact hole is made in a later step.

It should be noted that the film thus used is not limited to the siliconnitride film 26; however it is necessary to use a film having anelectrical insulating property for the following reason. If a conductivefilm made of, for example, polycrystalline silicon is used for thesilicon nitride film 6, the film is set in an electrically floatingstate in the end, and such a state may cause the occurrence of a leakcurrent, which is not desirable for the element isolation. Or, if acontact to be formed later is displaced and is brought in contact withthe film, and another contact of an adjacent element is displaced in thediffusion layer such as to be in contact with the film, the diffusionlayers of these elements adjacent to each other are electricallyconnected to each other via the film which extends to the inner sidesurface of the element isolation section, which causes a problem to theoperation of the semiconductor device.

Next, as shown in FIG. 2C, for example, the silicon nitride film 26 isetched back by anisotropic etching. In this manner, the remainingportion of silicon nitride film 26′ (to be called side wall hereinafter)is left selectively only on the lateral surface (inner wall) portion ofthe buried element isolation trench 24.

Further, boron ions are implanted to the above-described structure at anacceleration voltage of 20 keV and a concentration of about 1×10¹³ cm⁻²in the case where the substrate (or well) 21 in the region where theburied element isolation trench 24 is formed is p-type. Or, for example,phosphor ions are implanted to the above-described structure at anacceleration voltage of 30 keV and a concentration of about 1×10¹³ cm⁻²in the case where the substrate (or well) 21 in the region where theburied element isolation trench 24 is formed is n-type. Thus, in aregion of the substrate (or well) 21, which corresponds to the bottomportion of the buried element isolation trench 24, a punch-throughsuppression region 28 having the same conductivity type as that of thesubstrate (or well) 21 of the region and having an impurityconcentration higher than that of other substrate (or well) 21 locatedclose thereto, is formed.

Consequently, in the case of the structure shown in FIG. 2C, thepunch-through suppression region 28 made so as to control thepunch-through between element isolation regions is formed in a portionof the bottom surface 24 a of the buried element isolation trench 24,which is located on an inner side from the edge of the bottom surface 24a by a distance determined on the basis of the thickness of the film ofthe side wall 26′. It should be noted that in the above-described case,the thickness of the silicon nitride film is set to be larger than thedistance which the impurities diffuse, by a thermal step carried outafter the ion implantation.

Further, with regard to the structure shown in FIG. 2C, the insulatingfilm 29 is flattened by a CMP method or a resist etch back, after theinsulating film (filler member) 29 which is, for example, a siliconoxide film, is buried. Subsequently, the insulating film 29, the siliconnitride films 23 and 26, and the silicon oxide film 22 are removedexcept for what is inside the buried element isolation trench 24. Thus,as shown in FIG. 2D, a buried type element isolation structure 30, whichis made of the insulating film 29 and the side wall 26′, is completed.Next, as shown in, for example, FIG. 2E, a p-type well 32 a is formed inthe region in which the n-type transistor is formed, and similarly, ann-type well 32 b is formed in the region in which the p-type transistoris formed. After that, ion implantation designed for the adjustment ofthe threshold value is carried out so that the transistor has desiredelectrical characteristics.

A gate oxide film 33 is formed on the semiconductor substrate 21, and agate electrode 34 is formed on the gate oxide film 33.

Further, arsenic ions are implanted to the above-described structure atan acceleration voltage of 40 keV and a concentration of about 2×10¹⁵cm⁻² in the case where the substrate (or well) 1 in the region where thewell of the region in which the gate electrode 34 is formed is p-type(that is, in the case of a p-type well 32 a). Or, for example, BF₂ ionsare implanted to the above-described n-type well 32 b at an accelerationvoltage of 30 keV and a concentration of about 3×10¹⁵ cm⁻². In thismanner, a high-concentration diffusion layer region 31 which give riseto a source or drain region of an MOSFET is formed.

Next, as shown in FIG. 2F, an interlayer insulating film (firstinterlayer film) 35 made of, for example, silicon oxide film, isdeposited on the entirety of the semiconductor substrate 21. Then, theinterlayer insulating film 35 is selectively removed to form a contacthole 36 a in the region designed for the electrical connection. Further,a conductive material is filled into the contact hole 36 a, thus forminga contact 36 which is connected to the high-concentration diffusionlayer region 31. Further, a first wiring 37 connected to the contact 36is formed.

If necessary, further interlayer insulating films (the second, third, .. . interlayer films) and an upper layer wiring (any of them are notshown), or the like are formed. Then, lastly, a protection film 39 suchas a silicon nitride film is applied on the surface, and thus asemiconductor device having a plane structure shown in, for example,FIG. 3, is completed. Note that FIGS. 2A to 2F are cross sections takenalong the line II—II of FIG. 3.

In the buried type element isolation structure 30 prepared by theabove-described steps, the high-concentration diffusion layer region 31and the punch-through suppression region 28 having a high concentrationare formed without being contact with each other. Thus, in such ashallow and fine element isolation structure (so-called STI), thecapacitance of the high-concentration diffusion layer region 31 can beset to a desired value which is determined by the concentration of thesubstrate 21 or well 32 a or 32 b, thus making it possible to improvethe performance of the semiconductor device.

Further, the side wall 26′ which is a part of the silicon nitride film26 is present on the lateral surface of the element isolation trench 24.Therefore, even if the contact hole 36 a is displaced by some productionerror, the occurrence of the junction leak, which is caused by theetching of the element isolation structure, can be prevented. Therefore,the margin from the contact hole 36 a to the element isolation region(element isolation structure 30) can be shortened, and further itbecomes possible to form a contact hole 36 a in a self-alignment mannerwith respect to the element isolation region. This makes it possible tofurther downsize the devices.

Lastly, it should be noted that the first embodiment has been describedin connection with an example where the silicon nitride film 26 issubjected to anisotropic etching as illustrated in FIG. 2C, after thestep shown in FIG. 2B. However, the present invention is not limited tosuch a case, but it is possible that the step shown in FIG. 2B can befollowed directly by the step shown in FIG. 2D without exposing thebottom surface 24 a of the buried-type element isolation trench 24. Inthis case, the contact 36 to be connected to the high-concentrationdiffusion layer region 31 can be formed perfectly in a self-alignedmanner with respect to the element isolation structure 30.

(Second Embodiment)

FIG. 4, FIGS. 5A to 5E and FIG. 6 briefly illustrate the semiconductordevice according to the second embodiment of the present invention,taking the example where the product is an SRAM. FIG. 4 is a plan viewshowing the SRAM with a see-through image of the main portion thereof,FIGS. 5A to 5E are cross sectional views illustrating the steps of themanufacturing process of the SRAM, taken along the line V—V of FIG. 4,and FIG. 6 is a cross sectional view showing the structure of the SRAMtaken along the line VI—VI of FIG. 6.

First, as shown in FIG. 5A, a silicon oxide film 42 is formed to have athickness of about 10 nm, on a semiconductor substrate 41 by a thermaloxidation method or the like. Then, a polycrystalline silicon film (maskfilm) 43 is deposited to have a thickness of about 200 nm, on thesilicon oxide film 42 by a chemical vapor growth method or the like.

With respect to the above-described structure, the polycrystallinesilicon film 43, the silicon oxide film 42 and the semiconductorsubstrate 41 are subjected to anisotropic etching one after another by aphoto-etching method. Thus, a buried type element isolation trench 44having a tapered shape with a predetermined angle, which is a trenchused for isolating the buried type elements, is made. After that, heatoxidation is carried out, and consequently, a silicon oxide film 5having a thickness of, for example, about 15 nm is formed on the innerwall of the buried element isolation trench 44.

Next, with regard to the resultant structure shown in FIG. 5A, a siliconnitride film 46 is deposited to have a thickness of about 50 nm, on thesilicon nitride film 43 and the silicon oxide film 45, by a chemicalvapor growth method, as illustrated shown in FIG. 5B. The siliconnitride film 46 becomes to have an etching selection ratio with respectto the silicon oxide film 45 or an interlayer insulating film, when acontact hole is made in a later step, as in the case of the firstembodiment.

Next, as shown in FIG. 5C, for example, the silicon nitride film 46 isetched back by anisotropic etching. In this manner, the remainingportion of silicon nitride film 47 (to be called side wall hereinafter)is left selectively only on the lateral surface (inner wall) portion ofthe buried element isolation trench 44.

Further, boron ions are implanted to the above-described structure at anacceleration voltage of 20 keV and a concentration of about 1×10¹³ cm⁻²in the case where the substrate (or well) 41 in the region where theburied element isolation trench 44 is formed is p-type. Or, for example,phosphor ions are implanted to the above-described structure at anacceleration voltage of 30 keV and a concentration of about 1×10¹³ cm⁻²in the case where the substrate (or well) 41 in the region where theburied element isolation trench 44 is formed is n-type. Thus, in aregion of the substrate (or well) 41, which corresponds to the bottomportion of the buried element isolation trench 44, a punch-throughsuppression region 48 having the same conductivity type as that of thesubstrate (or well) 41 of the region and having an impurityconcentration higher than that of other substrate (or well) 41 locatedclose thereto, is formed.

Consequently, in the case of the structure shown in FIG. 5C, thepunch-through suppression region 48 made so as to control thepunch-through between element isolation regions is formed in a portionof the bottom surface 44 a of the buried element isolation trench 44,which is located on an inner side from the edge of the bottom surface 44a by a distance determined on the basis of the thickness of the film ofthe side wall 47. It should be noted that in the above-described case,the thickness of the silicon nitride film 46 which give rise to the sidewall 47 is set to be larger than the distance which the impuritiesdiffuse, by a thermal step carried out after the ion implantation.

Further, with regard to the structure shown in FIG. 5C, the insulatingfilm 49 is flattened by a CMP method or a resist etch back, after theinsulating film (filler member) 49 which is, for example, a siliconoxide film, is buried. Subsequently, the insulating film 49, the siliconnitride films 46, (side wall 47), the polycrystalline silicon film 43and the silicon oxide film 42 are removed except for what is inside theburied element isolation trench 44. Thus, as shown in FIG. 5D, a buriedtype element isolation structure 50, which is made of the insulatingfilm 49 and the side wall 47, is completed.

Next, as shown in FIG. 5D, for example, a p-type well 52 is formed inthe region in which the n-type transistor is formed, and similarly, ann-type well (not shown) is formed in the region in which the p-typetransistor is formed. After that, ion implantation designed for theadjustment of the threshold value is carried out so that the transistorhas desired electrical characteristics.

After that, as shown in FIG. 6, for example, a laminate structure of agate insulating film 53, a polysilicon layer (gate electrode) 54 and asilicon nitride film 55, is formed on the surface portion of thesemiconductor substrate 41. Then, ion implantation and thermalprocessing are carried out, to form a first source-drain diffusionregion (shallow diffusion layer) 41 a on the surface of thesemiconductor substrate. Subsequently, on both sides of the laminatestructure made of the gate insulating film 53, the polysilicon layer 54and the silicon nitride film 55, gate side wall 56 are formed. Asfurther ion implantation is carried out, a second source-drain diffusionregion (deep diffusion layer) 41 b is formed.

Next, with respect to the structure as shown in FIG. 6, an interlayerinsulating film (interlayer film) 57 made of, for example, silicon oxidefilm, is deposited on the entirety of the semiconductor substrate 41.Then, the interlayer insulating film 57 is selectively removed to form acontact hole 58 a in the region designed for the electrical connection.Further, a conductive material is filled into the contact hole 58 a,thus forming a contact 58 which is connected to the source-draindiffusion layer regions 41 a and 41 b. Further, a metal wiring layer 59connected to the contact 58 is formed, and thus SRAMs having crosssectional structures shown in FIG. 5E and FIG. 6 are completed.

The structure shown in FIG. 5E is a cross section of the SRAM shown inFIG. 4, taken along the line V—V, and the structure shown in FIG. 6 is across section of the SRAM shown in FIG. 4, taken along the line VI—VI.

In the case where the allowance between the contact hole 58 a and theelement isolation structure 50 is small, the contact hole 58 a extend tothe element isolation structure 50 in some cases due to the alignmenterror of the mask during the photo-etching, as shown in FIG. 5E. Even inthe above-described situation, the side wall 47 which is a part of thesilicon nitride film 46 is present in the outer periphery of the elementisolation structure 50 in the second embodiment of the presentinvention. Therefore, when an etching method which has a selectivitytowards the nitrogen film is used to open the contact hole 58 a, theelement isolation structure 50 is not etched. Therefore, the occurrenceof the junction leak can be prevented.

As an alternative version of the second embodiment, it is possible toform a punch-through suppression region 48 in the step shown in FIG. 5B,rather than in the step shown in FIG. 5C. In this version, boron ionsare implanted to the above-described structure at an accelerationvoltage of 40 keV and a concentration of about 1×10³ cm⁻² in the casewhere the substrate (or well) 41 in the region where the buried elementisolation trench 44 is formed is p-type. Or, for example, phosphor ionsare implanted to the above-described structure at an accelerationvoltage of 60 keV and a concentration of about 1×10³ cm⁻² in the casewhere the substrate (or well) 41 in the region where the buried elementisolation trench 44 is formed is n-type. Thus, a punch-throughsuppression region 48 can be easily formed without exposing the bottomsurface 44 a, by increasing the acceleration energy at the ionimplantation by an amount which corresponds to the thickness of thesilicon nitride film 46 deposited on the bottom portion of the buriedelement isolation trench 44.

(Third Embodiment)

FIGS. 7A to 7D briefly illustrate the process of manufacturing asemiconductor device, according to the third embodiment of the presentinvention.

First, for example, as shown in FIG. 7A, a silicon oxide film 62 isformed to have a thickness of about 10 nm, on a semiconductor substrate61 by a thermal oxidation method or the like. Then, a silicon nitridefilm (mask film) 63 is deposited to have a thickness of about 200 nm, onthe silicon oxide film 62 by a chemical vapor growth method or the like.

With respect to the above-described structure, the silicon nitride film53, the silicon oxide film 62 and the semiconductor substrate 61 aresubjected to anisotropic etching one after another by a photo-etchingmethod. Thus, a buried type element isolation trench 64 having a taperedshape with a predetermined angle, which is a trench used for isolatingthe buried type elements, is made. After that, heat oxidation is carriedout, and consequently, a silicon oxide film 65 having a thickness of,for example, about 15 nm is formed on the inner wall of the buriedelement isolation trench 64.

Next, with regard to the resultant structure shown in FIG. 7A, a siliconnitride film 66 is deposited to have a thickness of about 50 nm, on thesilicon nitride film 63 and the silicon oxide film 65, by a chemicalvapor growth method, as illustrated shown in FIG. 7B. The siliconnitride film 66 becomes to have an etching selection ratio with respectto the silicon oxide film 65 or an interlayer insulating film, when acontact hole is made in a later step, as in the case of the first orsecond embodiment.

Further, with respect to this structure, boron ions are implanted to theabove-described structure at an acceleration voltage of 40 keV and aconcentration of about 1×10¹³ cm⁻² in the case where the substrate (orwell) 61 in the region where the buried element isolation trench 64 isformed is p-type. Or, for example, phosphor ions are implanted to theabove-described structure at an acceleration voltage of 60 keV and aconcentration of 1×10¹³ cm⁻² in the case where the substrate (or well)61 in the region where the buried element isolation trench 64 is formedis n-type. Thus, in a region of the substrate (or well) 61, whichcorresponds to the bottom portion of the buried element isolation trench64, a punch-through suppression region 68 having the same conductivitytype as that of the substrate (or well) 61 of the region and having animpurity concentration higher than that of other substrate (or well) 61located close thereto, is formed.

Consequently, in the case of the structure shown in FIG. 7B, thepunch-through suppression region 48 made so as to control thepunch-through between element isolation regions is formed in a portionof the bottom surface 64 a of the buried element isolation trench 64,which is located on an inner side from the edge of the bottom surface 64a by a distance determined on the basis of the thickness of the siliconnitride film 66 formed on the inner wall of the element isolation trench64.

Further, with regard to the structure shown in FIG. 7B, the insulatingfilm 69 is flattened by a CMP method or a resist etch back, after theinsulating film (filler member) 69 which is, for example, a siliconoxide film, is buried. Subsequently, the insulating film 69, the siliconnitride films 63 and 66, the silicon oxide film 62 are removed exceptfor what is inside the buried element isolation trench 64. Thus, asshown in FIG. 7C, a buried type element isolation structure 70, which ismade of the insulating film 69 and the silicon nitride film 66, iscompleted.

Next, as shown in FIG. 7C, for example, a p-type well 72 is formed inthe region in which the n-type transistor is formed, and similarly, ann-type well (not shown) is formed in the region in which the p-typetransistor is formed. After that, ion implantation designed for theadjustment of the threshold value is carried out so that the transistorhas desired electrical characteristics.

Next, as in the second embodiment, an interlayer insulating film 73 madeof, for example, silicon oxide film, is deposited on the entirety of thesemiconductor substrate 61 after the formation of the gate (not shown)of the MOS transistor and the diffusion layer region 71. Then, theinterlayer insulating film 73 is selectively removed to form a contacthole 74 a in the region designed for the electrical connection. Further,a conductive material is filled into the contact hole 74 a, thus forminga contact 74 which is connected to the diffusion layer region 71. Thus,a semiconductor device having a cross sectional structure such as shownin FIG. 7D is completed.

In this embodiment, if the allowance between the contact hole 74 a andthe element isolation structure 70 is small, the contact hole 74 aextend to the element isolation structure 70 in some cases due to thealignment error of the mask during the photo-etching, as shown in FIG.7D. Even in the above-described situation, the silicon nitride film 66is present in the outer periphery of the element isolation structure 50in the third embodiment of the present invention. Therefore, when anetching method which has a selectivity towards the nitrogen film is usedto open the contact hole 74 a, the element isolation structure 70 is notetched. Therefore, the occurrence of the junction leak can be prevented.

It should be noted that in the third embodiment, the buried elementisolation trench is made into a tapered shape, and the angle of thetaper is set in consideration of the allowance for case where thecontact hole 74 a is formed to be displaced on the element isolationstructure 70. Further, with the tapered shape of the buried elementisolation trench 64, it becomes easy to set the thickness of the siliconnitride film 66 formed on the inner wall of the element isolation trench64 to a desired value. However, the adjustment, that is, an increase ordecrease, of the taper angle is determined by the limitation of the fineprocess, and a preferable angle is about 60° to 85°.

Further, in any of the first to third embodiments, it is not necessaryto form all the contacts to extend over the diffusion layer region andthe buried element isolation structure. In other words, it suffices onlyif at least one contact is formed to extend over the diffusion layerregion and the buried element isolation structure.

Apart from the above, it is natural that the preset invention can beremodeled into various versions of embodiments as long as the essence ofthe invention remains.

As described above in detail, with the present invention, it becomespossible to form a buried element isolation structure without bringingthe high-concentration diffusion layer and a relatively highconcentration region made for the purpose of the punch-throughsuppression in contact with each other. Therefore, a semiconductordevice, even if it has a shallow or fine element isolation structure,can be made capable of a high performance, and preventing the occurrenceof a junction leak between the high-concentration diffusion layer regionand the well. Thus, the present invention can provide a semiconductordevice having a buried type element isolation structure, capable of highperformance, and a method of manufacturing such a semiconductor device.

Moreover, the capacitance of the high-concentration diffusion layerregion can be set to a desired value which is determined by theconcentration of the well, thus making it possible to improve theperformance of the semiconductor device. Further, the silicon nitridefilm is present on the lateral surface of the element isolation trench.Therefore, even if the contact hole is displaced by some productionerror, the occurrence of the junction leak, which is caused by theetching of the element isolation structure, can be prevented. Therefore,the margin from the contact hole to the element isolation region can beshortened, and further it becomes possible to form a contact hole in aself-alignment manner with respect to the element isolation region. Thismakes it possible to further downsize the products.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modification may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. A method of manufacturing a semiconductor devicehaving a buried-type element isolation structure, comprising: forming aburied-type element isolation trench in a semiconductor substrate of afirst conductivity type; forming a first insulating film on an innersurface of the buried-type element isolation trench; forming a secondinsulating film having an etching selectivity ratio to an interlayerfilm on a side wall portion of said first insulating film formed on theinner surface of the buried-type element isolation trench; introducingimpurities through said first insulating film to a bottom portion of theburied-type element isolation trench to form a high-concentrationimpurity region of the first conductivity type under the bottom portionof the buried-type element isolation trench in said semiconductorsubstrate so that the high-concentration impurity region contacts to thefirst insulating film at the bottom portion of said buried-type elementisolation trench with a distance determined by a thickness of the firstinsulating film; and burying an insulation material in said buried-typeelement isolation trench to form a buried-type element isolationstructure in said buried-type element isolation trench together with thesecond insulating film; forming a diffusion region of a secondconductivity type in a surface portion of the semiconductor substrateexcept for a region where said buried-type element isolation trench isformed; depositing said interlayer insulation film on a whole surface ofsaid semiconductor substrate; and forming a contact section through theinterlayer insulation film so that the contact section is connected tothe diffusion layer and is formed to extend over the diffusion layer andthe buried-type element isolation structure.
 2. A method ofmanufacturing a semiconductor device according to claim 1, furthercomprising: performing a thermal process for thermally diffusing theintroduced impurities to an extent equal or small than the thickness ofthe first insulating film.
 3. A method of manufacturing a semiconductordevice, according to claim 1, wherein said second insulating filmforming comprises: depositing an insulation layer on a whole inner wallportions including the side wall portion and a bottom wall portion; andperforming anisotropic etching the insulation layer so that theinsulation layer deposited on the bottom wall portion is removed and thesecond insulating film is partially left on only the side wall portionof the first insulating film formed on the buried-type element isolationtrench.
 4. A method of manufacturing a semiconductor device, accordingto claim 3, wherein said insulation layer is formed by depositing asilicon nitride film on the first insulating film formed on the innerwall of the buried-type element isolation trench.
 5. A method ofmanufacturing a semiconductor device, comprising: forming an oxide filmon a semiconductor substrate of a first conductivity type; forming amask on said oxide film to form a buried-type element isolation trenchin said semiconductor substrate; forming said buried-type elementisolation trench in said semiconductor substrate by performinganisotropic etching the oxide film and the semiconductor substrate usingsaid mask; forming an insulating film along an inner surface of theburied-type element isolation trench; forming a high-concentrationimpurity region of the first conductivity type at a region beneath theinsulating film formed at a bottom surface of the buried-type elementisolation trench; forming a buried-type element isolation structure byburying a filler insulation member in the buried-type element isolationtrench; forming a diffusion layer of a second conductivity type in asurface region of said semiconductor substrate except for a region wheresaid buried-type element isolation structure is formed; depositing aninterlayer film having an etching selectivity ratio to the insulatingfilm on an entire surface of said semiconductor substrate; and forming acontact section pierced through the interlayer film to be connected tothe diffusion layer in a self-alignment manner with respect to theburied-type element isolation structure.
 6. A method of manufacturing asemiconductor device according to claim 3, wherein said contact sectionis formed to extend over said diffusion layer and said buried-typeelement isolation structure.
 7. A method of manufacturing asemiconductor device according to claim 5, wherein said buried-typeelement isolation trench is formed to have a predetermined taper anglewith respect to a surface of said semiconductor substrate.
 8. A methodof manufacturing a semiconductor device according to claim 5, furthercomprising: performing a thermal process for thermally diffusing thehigh-concentration impurity region to an extent equal or small than thethickness of the insulating film.
 9. A method of manufacturing asemiconductor device according to claim 5, which further comprises:depositing an insulation layer on a whole inner surface of theinsulating film formed on the buried-type element isolation trench;etching selectively a portion of the insulation layer formed on a bottomportion of the buried-type element isolation trench so as to remain aside wall insulation layer on a side wall of the inner surface of theinsulating film.
 10. A method of manufacturing a semiconductor deviceaccording to claim 9, wherein said high-concentration impurity region isformed while said insulation layer is selectively left on only the sidewall portion of the buried-type element isolation trench via theinsulating film.
 11. A method of manufacturing a semiconductor deviceaccording to claim 9, wherein a silicon nitride film is deposited assaid insulation layer.